In a transmitter based on an amplitude modulation system, when any trouble such as a failure of a circuit occurs, abnormal modulation is executed, which may result in transmission of unnecessary electric waves. Transmission of unnecessary electric waves give undesirable effects over other communications.
FIG. 6 shows configuration of a transmitter based on an orthogonal modulation system which is a sort of amplitude modulation system. An aural signal inputted from a microphone 1 is amplified by an microphone amplifier 2, and then is converted to an A/D convertor 3. The digital signal is processed in a base band signal processing circuit 4, and then converted to an analog signal by a D/A convertor 5. The analog signal is amplified by a modulation amplifier 6, and then is subjected to an orthogonal modulation by an orthogonal modulator 7. The signal to be modulated is amplified by an transmission amplifier 8, and the electric waves are transmitted via a directive coupler 9 from an antenna 10.
A distortion reducing circuit 11 comprises a demodulator 12 and an amplifier 13, takes out only progressive waves from the transmission amplifier 8 by using a directive coupler 9, demodulates and amplifies the progressive waves, and then feedbacks the waves to the modulation amplifier 6 to reduce distortion of the transmitted signal.
A transmission local (carrier) is added via a power distributor 14 to the modulator 7 and the demodulator 12 respectively.
In a transmitter employing an orthogonal modulation system therein, as it is important to accurately maintain a phase, only progressive waves each not including reflected waves are taken out from a signal fed back through the distortion reducing circuit 11, and for this reason the directive coupler 9 is used.
In a transmitter .based on the orthogonal modulation system as described above, when the transmitter is working in the normal state, distortion of a transmitted signal is small, and width of the spectrum is extremely small.
FIG. 7 shows a spectrum of a transmitted signal based on the orthogonal modulation system. When the transmitter is working in the normal state, distortion of a signal is small, a spectrum (indicated by a solid line) at a point P generated by an modulated signal when it is assumed that the signal to be modulated by two carriers crossing each other at right angles comprises an I modulated signal and a Q modulated signal is extremely small. When a fault occurs in the transmitter, an electric power at point P generated according to the I modulated signal increases as shown by the dotted line. Namely the spectrum becomes wider. This change is detected by an abnormal transmission stopping circuit 20, a transmitting function of the transmitter is stopped, and transmission of unnecessary electric waves due to a fault of operation of the circuit is prevented.
Concretely the abnormal transmission stopping circuit 20 comprises a differentiation circuit 15, an amplifier 16, a detector circuit 17, and a comparator 18, differentiates output from the amplifier 13 in the differentiation circuit 15, amplifies the output with the amplifier 16, detects the output in the detector circuit 17, and inputs the electric waves into the comparator 18. In the comparator 18, when output from the detector circuit 17 exceeds a threshold value, it is regarded that the spectrum is widening, and a transmitting function stopping signal is generated with operation of the base band signal processing circuit 4 stopped.
The conventional type of abnormal transmission stopping circuit as described above has the shortcomings as described below.
(1) The circuit configuration is complicated.
(2) In such a case as where a modulating function is accidentally disabled with only a balance of an orthogonal modulator lost and only a transmission local (carrier) is transmitted with a full power, the faulty state of the transmitter can not be detected because the spectrum is not expanded. In this case, unnecessary electric waves are transmitted and other communications are interfered.